Approximately one-third to two-thirds the cost of a barrel of oil is spent in drilling. This represents $200 million per day for world production of 60 million barrels per day, at $10/bbl production cost. The time spent drilling hydrocarbon wells in rotating the drill bit at the bottom of the hole (i.e., actually cutting the rock) increases from about 25% for shallow wells to almost 50% as the well depth increases to 15,000 feet, with an overall average of 34%. Of the total footage drilled, 75% is in shales. These figures imply, in the most conservative case, that world expenditures while drilling shales are in excess of $20 million per day. Drilling through shales also causes over 90% of wellbore stability problems. The drilling of shales can result in a variety of problems from washout to complete collapse of the hole. More typically, drilling problems in shales are bit balling, sloughing, or creep. The problem is severe; it as been estimated to be a $500 million/year problem.sup.1-5.
The engineering problems of instability in shales are closely connected with the bulk properties of shales such as strength and deformation which are a function of depositional environment, porosity, water content, clay content, composition, compaction history, etc. The bulk properties of the drilling fluid such as the chemical makeup and concentration of the continuous phase of the mud, the composition and type of an internal phase if present, the additives associated with the continuous phase, and the maintenance of the system are also of engineering importance. Other factors such as in situ stresses, pore pressure, temperature, time in open hole, depth and length of the open hole interval, surrounding geological environment (salt dome, tectonics, etc.), also directly impact drilling and completion operations. For a successful drilling operation these parameters must be integrated into well planning, mud system selection criteria, and/or new mud development. These variables are interconnected and influence the overall (in)stability in shales while drilling.
As oil reserves deplete and the cost of drilling increases, the need to drill extended-reach wells with long open hole intervals will also increase. In the past, oilbased muds (OBM) have been the workhorse of the industry for difficult drilling. Their application has been typically justified on the basis of borehole stability, fluid loss, filtercake quality, lubricity, and temperature stability. As the environmental concerns restrict the use of oil-based muds, the industry must provide innovative means to obtain OBM performance without negatively impacting the environment. The successful introduction and application of ester-based biodegradable invert emulsion drilling fluids in the past decade have provided attractive alternatives to traditional OBM in accessing hydrocarbon reserves located in environmentally sensitive regions. The costs associated with the use of these biodegradable invert emulsion drilling fluid systems limit the application of such systems on a routine basis. Water-based muds (WBM) are attractive replacements from a direct cost point-of-view. But, conventional WBM systems have failed to meet key performance criteria obtained with OBM in terms of rate of penetration (ROP), bit and stabilizer balling, lubricity, filtercake quality and thermal stability. More importantly, severe borehole (in)stability problems are encountered when drilling shale formations with conventional WBM, leading to significant increases in the overall well cost.
Past efforts to develop improved WBM for shale drilling have been hampered by a limited understanding of the drilling fluid/shale interaction phenomenon. This limited understanding has resulted in drilling fluids designed with non-optimum properties required to prevent the onset of borehole instability. Historically, problems have been approached on a trial-and-error basis, going through a costly multiwell learning curve before arriving at reasonable solutions for optimized operations and systems. Recent studies.sup.1-4,6,9 of fluid-shale interactions, however, have produced fresh insights into the underlying causes of borehole (in)stability, leading to the design of water-based, shale drilling fluids in accord with the present invention.
The following analysis summarizes the complex phenomenon of the physcochemical interaction between drilling fluids and shale,and provides strategies to apply this understanding to the design of WBM to combat borehole instability problems in shales. A theoretical framework based on fundamental thermodynamic principles conceptualizes the relationship between different driving forces (hydraulic and chemical potential) and critical drilling fluid/shale system parameters.